1. Field of the Invention
The present invention relates to a process for producing carboxylic acid esters from aldehydes and alcohols by one-step reaction, and a catalyst used in such process. More particularly, the invention pertains to a process for producing methacrylic esters or acrylic esters of high industrial value from methacrolein or acrolein in an economically advantageous way, and a catalyst used in this process.
2. Description of the Related Art
For the production of carboxylic acid esters such as methacrylic esters and acrylic esters, there has been available an industrially established process in which methacrolein or acrolein, which is an aldehyde, is subjected to gas phase catalytic oxidation to produce a methacrylic or acrylic acid which is a carboxylic acid, and it is further subjected to an esterification reaction with an alcohol to produce a methacrylic or acrylic ester.
This process, however, has disadvantages in that it necessitates large production equipment as this process involves two-step (oxidation and esterification) reaction operations. Another problem of this process is poor performance of the catalyst used in the reactions. In production of a methacrylic ester according to this process, a heteropoly-acid salt type catalyst composed mainly of phosphorus and molybdenum is used in the step of oxidizing methacrolein to produce methacrylic acid, but this catalyst is unsatisfactory in durability and yield of product.
Studies are underway on the methods for producing carboxylic acid esters in a high yield by one-step liquid phase reaction from an aldehyde and an alcohol. Various catalysts have been proposed for use in these methods. For example, JP-B-57-35856 proposes Pd- and Pb-based catalysts using calcium carbonate as carrier, JP-B-4-72578 proposes Pb-based catalysts using zinc oxide as carrier, JP-A-57-50545 and JP-A-61-243044 propose various types of Pd- and Pb-based catalysts, JP-B-61-60820 proposes Pd- and Bi-based catalysts, JP-B-62-7902 and JP-A-5-148184 propose the catalysts composed Pd and at least one element selected from Pb and Bi, etc., JP-B-57-35860 proposes Pd-, Tl- and Hg-based catalysts, and JP-B-57-19090 proposes Pd-, alkaline earth metal-, Zn- and Cd-based catalysts.
The Pd-based catalysts decreasse in activity in long-time use since Pd in the catalyst is oxidized in the reaction, so that these catalysts need to be regenerated for continuous use. As means for activating the catalysts, there is proposed, in JP-A-9-38506 for instance, use of formalin, formic acid, hydrazine, methanol and molecular hydrogen in the presence of a Pb-containing substance in preparation of Pd- and Pb-based catalysts. This method, however, is only applicable to preparation of a new catalyst; there is known no established method for regenerating for reusing a catalyst which has decreased in activity in use for reactions.
The carboxylic acid ester production processes using a catalyst such as mentioned above also have the following problems: the reaction rate is low; the objective carboxylic acid ester is obtained as a low-concentration alcohol solution; many by-products such as methyl formate are formed; catalyst life is short; yield of the product is unsatisfactory. Thus, further improvements of these processes have been desired.